Cathode for electrolytic production of titanium and other metal powders

ABSTRACT

Disclosed herein are electrolytic cells comprising cathodes having a non-uniform current distribution and methods of use thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/765,560, filed Feb. 6, 2006 which is incorporated by reference hereinin its entirety.

FIELD OF THE INVENTION

Disclosed herein are electrolytic cells comprising cathodes having anon-uniform current distribution and methods of use thereof for theproduction of titanium and other multi-valence and high (2 or more)valence metals, in particular refractory metals such as, for example,chromium, hafnium, molybdenum, niobium, tantalum, tungsten, vanadium,and zirconium.

BACKGROUND OF THE INVENTION

The simplest electrolytic cell for use in electrowinning metals consistsof at least two electrodes and a molten electrolyte. The electrode atwhich the electron producing oxidation reaction occurs is the anode. Theelectrode at which an electron consuming reduction reaction occurs isthe cathode. The direction of the electron flow in the circuit is alwaysfrom anode to cathode.

Metal particles are removed from solid cathodes by force of gravity orforced fluid flow across the face of the cathode. If the metal particlesgrow too large or strongly stick to the surface of the cathode, theparticles are difficult to dislodge and collect. Other levers thatpeople have found to control particle size and morphology include: 1.feedstock concentration, 2. temperature, 3. electrolyte compositionincluding special additives, and 4. current density.

Thus, there remains a need to control metal particle size via cathodedesign. It an objective herein to design electrolytic cells to producepowders fulfilling these needs through cathode design and fluid flowwhich control the cross-sectional area and height of particles growingfrom the surface of the cathode.

SUMMARY OF THE INVENTION

One aspect is for electrolytic cell comprising a cathode having anon-uniform current distribution.

A further aspect is for a method of controlling the morphology of ametal product in an electrolytic cell comprising the steps of (a)providing an electrolytic cell comprising a molten electrolyte, acathode having a non-uniform current distribution in contact with themolten electrolyte, and an anode in contact with the molten electrolyte;(b) providing a metal compound to the electrolyte; and (c) applyingeither a fixed voltage or a fixed current across the anode and thecathode thereby depositing metal on the cathode.

Other objects and advantages will become apparent to those skilled inthe art upon reference to the detailed description that hereinafterfollows.

DETAILED DESCRIPTION OF THE INVENTION

Applicants specifically incorporate the entire content of all citedreferences in this disclosure. Further, when an amount, concentration,or other value or parameter is given as either a range, preferred range,or a list of upper preferable values and lower preferable values, thisis to be understood as specifically disclosing all ranges formed fromany pair of any upper range limit or preferred value and any lower rangelimit or preferred value, regardless of whether ranges are separatelydisclosed. Where a range of numerical values is recited herein, unlessotherwise stated, the range is intended to include the endpointsthereof, and all integers and fractions within the range. It is notintended that the scope of the invention be limited to the specificvalues recited when defining a range.

The disclosure herein, while relating in particular to the production oftitanium from a titanium oxide, is also applicable to the production oftitanium from other titanium compounds as well as for the production ofother metal compounds such as, for example, chromium, hafnium,molybdenum, niobium, tantalum, tungsten, vanadium, or zirconium from,for example, the respective oxides, halides, nitrides, or sulfides.

Cathode design is used to aid in controlling the cross-sectional area ofelectrodepositing metal particles by controlling the lines of constantpotential parallel to the face of the cathode surface. Isopotentiallines will be parallel to the contour of the surface of the electrodeand current distribution will be orthogonal or perpendicular to theselines and with metal deposition rates proportional to current density,the areas of highest current density will have the largest metaldeposition rates. Furthermore, current density is highest where thedistance between cathode and anode is shortest so as particles grow fromthe cathode, current densities at the tip of the growing particles arehighest.

One embodiment for producing this non-uniform current distributionrelates to a cathode comprising a wire mesh screen. If a screen is usedto control particle cross-sectional area, particles can only grow wherethere is metal mesh, so, for example if the mesh is 100 microns across,the cross-sectional area of the particles formed will have an averagecross-section of 100 microns. Similarly various other cathode sizes,shapes, and designs can be used to achieve the same non-uniform currentdistribution effect. For example, additional useful cathode designsinclude, but are not limited to, bristles, cones, rods, combinationsthereof, and combinations with mesh screens.

The height to which particles can grow from the cathode surface can becontrolled by adjusting the electrolyte flowrate so the fluid wouldshear particles as they form to the desired height. Alternatively,mechanical means can be used to dislodge the particles as they growtoward the anode, for example, vibration. Gas blowers can also be usedto dislodge the particles.

There are preferred particle size ranges, particle aspect ratio rangesand particle morphologies preferred for each powder metallurgicalprocessing method used to make various forms of metal parts. Forexample, in metal injection molding using small parts, symmetricspherical powders with particles less than 45 microns are preferred. Inpress and sintering, 45 to 150 micron asymmetric powder particles withaspect ratios of >1.5 are preferred. Those who desire to make thin sheetfrom powder prefer asymmetric particles with large aspect ratios and cantolerate wide size distributions above 45 microns.

Anodes useful in standard electrolytic cells can be utilized in anelectrolytic cell containing a cathode having a non-uniform currentdistribution. For example, carbon anodes, inert dimensionally stableanodes, or a gas diffusion anodes fed with a combustible gas are alluseful in electrolytic cells containing a cathode having a non-uniformcurrent distribution. Other useful anodes include consumable anodescontaining a compound of the metal, such as titanium, to be deposited atthe cathode. Consumable anodes are known in the art and an example of asuitable consumable anode is described in U.S. Pat. No. 2,722,509 whichis incorporated herein by reference. One anode or multiple anodes can beemployed. In one embodiment, the anode can be a molten metal anode asdisclosed in U.S. Patent Publication No. 2005/0121333, incorporatedherein by reference.

Typically, the metal compound to be electrowon is a metal oxide, forexample titanium oxide or titanium dioxide. It is also possible,however, to electrowin a metal from other metal compounds that are notoxides. These compounds include, for example, halides such as, e.g.,TiCl₃, nitrides such as, e.g., titanium nitride, and carbides such as,e.g., titanium carbide. The metal compound may be in the form of a rod,sheet or other artifact. If the metal compound is in the form of swarfor particulate matter, it may be held in a mesh basket. In anotherembodiment, the metal compound can also be solubilized in theelectrolyte, optionally with the assistance of standard solubilizers.

By using more than one metal compound, it is possible to produce analloy. The metal compounds for alloy production may be incorporated intothe molten electrolyte simultaneously, added stepwise, or in any othermanner as is necessary to produce the desired alloy. For example, analloy of Ti—Al—V can be produced by mixing aluminum oxide, vanadiumoxide, and TiO₂ in the electrolyte thereby to produce an alloy ofTi—Al—V in the molten zinc cathode. The E₀ and current density should beadjusted to deposit precise composition alloy particles.

The electrolyte consists of salts which are preferably more stable thanthe equivalent salts of the metal which is being deposited. Using saltswith a low melting point, it is possible to use mixtures if a fused saltmelting at a lower temperature is required, e.g. by utilizing a eutecticor near-eutectic mixture. It is also advantageous to have, as anelectrolyte, a salt with as wide a difference between the melting andboiling points, since this gives a wide operating temperature withoutexcessive vaporization. Exemplary electrolytes include, but are notlimited to, metal fluorides, metal chlorides, and mixtures thereof.

In one embodiment, the level of metal compound provided to the moltenelectrolyte is continuously adjusted in order to insure continuousoperating electrolysis.

All of the compositions and methods disclosed and claimed herein can bemade and executed without undue experimentation in light of the presentdisclosure. While the compositions and methods of this invention havebeen described in terms of preferred embodiments, it will be apparent tothose of skill in the art that variations may be applied to thecompositions and methods and in the steps or in the sequence of steps ofthe method described herein without departing from the concept, spirit,and scope of the invention. More specifically, it will be apparent thatcertain agents which are chemically related may be substituted for theagents described herein while the same or similar results would beachieved. All such similar substitutes and modifications apparent tothose skilled in the art are deemed to be within the spirit, scope, andconcept of the invention as defined by the appended claims.

1. A method of controlling the morphology of a titanium-containing product in an electrolytic cell comprising the steps of: (a) providing an electrolytic cell comprising a molten electrolyte, a cathode having a non-uniform current distribution in contact with the molten electrolyte, and an anode in contact with the molten electrolyte; (b) providing a titanium-containing compound to the molten electrolyte; and (c) applying either a fixed voltage or a fixed current across the anode and the cathode thereby depositing a metal comprising titanium on the cathode.
 2. The method of claim 1, comprising during or after step (c) the further step of removing the titanium from the cathode when the metal attains a morphology suitable for powder metallurgical applications.
 3. The method of claim 2, wherein the removing step is accomplished by fluid flow, vibration, or blown gas.
 4. The method of claim 2, wherein the metal is a low aspect ratio powder with a particle size of less than 45 microns.
 5. The method of claim 2, wherein the metal is asymmetric powder particle with a particle size in a range of from 45 to 150 micron.
 6. The method of claim 5, wherein the asymmetric powder particle has an aspect ratio of greater than 1.5.
 7. The method of claim 1, wherein the cathode is wire mesh, bristles, rods, or combinations thereof.
 8. The method of claim 1, wherein the metal is titanium.
 9. The method of claim 1, wherein step (b) is accomplished by adding the titanium-containing compound to the molten electrolyte.
 10. The method of claim 1, wherein the titanium-containing compound is titanium monoxide or titanium dioxide.
 11. The method of claim 1, wherein the anode is a consumable anode and the titanium-containing compound is a component of the consumable anode which is provided to the molten electrolyte 